Although I know it's not a universally accepted view, I personally don't believe that sins are arbitrarily chosen imperatives. I believe sins are things that will hurt your and/or others. When God tells you not to sin, he is not commanding you, he is warning you that what you are about to do will hurt you and/or others. God telling you not to sin is like a parent telling their child not to stick an object in the light socket.

If you agree with this view of sin then the question of whether something like marijuana is sinful or not becomes identical to the question of whether it hurts you, hurts others, and/or hurts society.

These are the questions I ask, in somewhat order of priority, when considering whether something is sinful:

1. Does the Bible talk about it?
2. Would it bring dishonor to God?
3. Will it hurt you?
4. Will it hurt others?
5. Would a virtuous person do such a thing? (This is almost identical to the question of whether the act goes against the beatitudes or the fruits of the spirit).
6. What impact would it have on the world if many people did that act?

What marijuana's status boils down to for me is questions 3 and 6.

Being a pothead is almost certainly sinful because of the destructive effect it has in people's lives. Using marijuana as a way to deal with your problems is, in addition to being one of the hallmarks of addiction, idolatry which adds a failure of test #1 to the list of reasons it is sinful.

Consuming marijuana to unwind before bed, or as a social lubricant, or enhancement, is probably fine.